The core components and subsystems of a typical single-panel LCOS projection system in a compact configuration include, in the order of the optical path, a light source, a polarizer, a polarizing beam splitter (PBS), an LCOS modulation imager, and last but not least, a projection lens. Both compactness and optical efficiency are highly expected for such a polarizer used in a pico projection system with limited allowance in both physical size and power consumption of the key components and overall system.
Unpolarized light includes a linear component and an orthogonal component. A common method for producing polarized light for such an LCOS projection system comprises the use of the PBS. The linearly polarized component light, herein named P polarization, is either transmitted or reflected by the PBS to the LCOS panel, while the orthogonal component, named S polarization, is either reflected away or transmitted unused in a perpendicular direction accordingly. This PBS also transmits or reflects modulated light by the LCOS imager in a rotated polarization state, from P polarization to S polarization or vice versa accordingly towards the projection lens for projection display. Theoretically, at least 50% of illumination is unused or discarded from the PBS by the system, thereby substantially limiting optical efficiency of the system in utilization of illumination from the light source.
A number of attempts have been made in the prior art to recycle the reflected unused polarization component in such a delivery subsystem of polarization light to illuminate the LCOS imager. Among those, plate-like element polarization converters are most attractive because of their most adequate applicability to a compact single-panel projection system.
An earlier plate-like polarization converter is disclosed in U.S. Pat. No. 5,566,367. Another planar polarization converter for polarizing a light beam is disclosed in U.S. Pat. No. 5,940,149.
FIG. 1a is a cross-section view illustrating a conventional plate-like polarization converter, and FIG. 1b is a cross-sectional view of the plate-like polarization converter of FIG. 1a further comprising a light deviator assembly in a back position and a beam integrating film in a front position along the light path. As shown in FIG. 1a, in the order of incidence along the optical path of the light beam defining front position and back position, the planar polarization converter includes a prismatic film 640, a polarization film sheet 660 and a reflective polarization film 670. The prismatic film 640 has a first prismatic surface 642. The first prismatic surface 642 has alternating transmissive prismatic facets 644 and reflective prismatic facets 646 positioned at supplementary base angles, each of the transmissive prismatic facets 644 is positioned at a first base angle v relative to a base plane 601, and each of the reflective prismatic facets 646 is positioned at a second base angle generally equal to 180°-v. The polarization film sheet 660 specifically may be a quarter wave retarder film positioned between the prismatic film 640 and the reflective polarization film 670. A deviated light beam 622 transmits through the transmissive prismatic facets 644 and the quarter wave retarder film 660, and a first light beam 624a of the incident light beam in a first polarization state P transmits through the reflective polarization film 670, and a second light beam 626 of the incident light beam in a second polarization state S is reflected by the reflective polarization film 670. The second light beam 626 is reflected by the reflective prismatic facets 646 after transmitting through the quarter wave retarder film 660, and then transmitting through the quarter wave retarder film 660 once again. Since the second light beam 626 transmits through the quarter wave retarder film 660 twice, the second light beam 626 is polarized to be a third light beam 624b in the first polarization state. The third light beam 624b could transmit through the reflective polarization film 670.
As shown in FIG. 1b, additional embodiments of the above planar polarization converter further include adaptation of a light alignment assembly 630 and a prismatic beam-integrating sheet 690. The light alignment assembly 630 consists of collimating elements and light deviating elements on a prismatic surface, and the prismatic beam-integrating sheet 690 comprises collimating elements on another prismatic surface. Such a planar polarization converter is aligned optically to receive and convert the unpolarized collimated light beam to linearly polarized light suitable for adequate use in a single-panel LCOS projector.
Further need for improvement in device integration and simplification in assembly robustness is recognized in at least two aspects among others, including the integration and handling of such a polarization converter assembly and optical efficiency of conversion in terms of transmission loss. The polarization converter disclosed by all the embodiments in U.S. Pat. No. 5,940,149 still requires a spatial assembly of three to five separate plate-like but non-optically-flat components in adequate optical alignment, which increases the complexity of its integration with a projection system. Besides, to enable challenging infield application, all the sub components are preferably made from solid state materials instead of polymeric substrates with improved robustness of material roughness and precision of fabrication and assembly.